Pipe is inserted into the earth to perform tasks that are as diverse as the simple job of establishing building footings to the more complex task of constructing a deep offshore well. The pipe may be jetted into place using a washing or jetting action, it may be drilled into the earth by rotating a bit or auger at the base of the pipe, it may be lowered into a pre-drilled bore hole or it may be pounded into the earth by repeated impact blows. Other methods of inserting pipe into the earth may also be employed.
As applied to well construction, the pipe may be used to drill the well, to case the well, to complete the well, to flow effluent from the well or to workover the well. Most pipe used in the construction of a well is formed into a long pipe string by securing together individual pipe sections, or “joints”, using some form of a threaded connector. The connector is typically in the form of an externally threaded member, sometimes referred to as a “pin” that is threadedly received within an internally threaded member, sometimes referred to as a “box”. The box may be integrally formed at one end of the pipe section or may be provided by the threaded engagement of a coupling with a pin formed at the end of the pipe section. In securing pipe sections together, a pin formed at an end of one pipe section is engaged within the box provided at an end of another pipe section. The type of thread used in the connector is determined primarily by the use to which the pipe is to be put.
Threaded pipe connections are usually assembled with the assistance of a thread lubricant that is applied to the threaded area to be engaged by the connection of the mating tubular bodies. The lubricant protects threads from corrosion and prevents galling and other mechanical thread damage during the assembly of the connection. In some connections, the lubricant also seals helical leakage paths formed between the engaged threads of the pin and box to make the threaded connection pressure-tight.
While the thread lubricant performs essential purposes in threaded connectors, its presence in certain types of threaded connections can also present problems. Some thread forms can trap the lubricant between the engaged surfaces of the pin and box threads. Assembly procedures and impact loading can also compress the trapped thread lubricant causing extremely high hydraulic forces that can damage the threaded connection. The presence of trapped lubricant in the pin and box can also prevent the threaded connection from being screwed together sufficiently to reach its optimum make up position.
Certain desirable thread designs and thread forms are not readily employed in applications where trapped thread lubricant may be subjected to high pressures. Wedge threads, dovetail threads, hooked threads and other complex thread forms that are well suited to perform desired mechanical holding and sealing functions in a threaded connector may be unsuitable for use in applications where trapped thread lubricant may become highly pressurized during the assembly or use of the connection.
Highly pressurized lubricant can also be problematic in the connectors used to assemble impact driven conductor pipe. Wells drilled in offshore bodies of water typically require the use of an external conductor pipe that extends from the floor of the body of water up to the drilling structure at the water surface. In deep waters, the conductor pipe is often installed by a process called “jetting”, which uses pumped fluids to remove enough soil to allow the conductor pipe to be lowered to the desired depth below the water bottom (“mud line”). Axial shock loading or impact are not typically applied to the pipe when the conductor pipe is jetted into place. Jetting, however, is not always an available method of installing the conductor pipe.
Offshore wells drilled from jack-up and platform rigs typically employ a conductor pipe that is driven to the desired depth or point of refusal by hammering on the pipe with large diesel or hydraulic hammers. As many as 30,000 or more hammer blows may be required to drive the conductor pipe 300 ft. below the mud line. Each hammer blow can apply up to 3 million lbs. of force to each connection making up the conductor string.
The conductor pipes used in constructing offshore wells are assembled from individual joints of pipe connected together at their ends by welding or by threaded connections. Weld lines are well able to withstand the high, cyclic impact delivered by the hammers because of the solid-state nature of welds. However, threaded or other mechanical connections used to secure conductor pipe sections together are not inherently resistive to repetitive, impact loading. Threaded connections to be used in these applications must be specially designed to withstand high, cyclic loading because of the stress risers inherently required to create the metal thread configuration providing the mechanical connection. The industry has addressed fatigue failures in threaded conductor drive pipe by expressly designing the connections with larger dimensions and stronger materials to withstand high fatigue resistance.
High Internal connection pressure within conventional impact driven conductor pipe has not typically been a problem because sealing of the connector against high Internal pipe pressures was not usually required. Historically, the conductor pipe string has not been subjected to applied internal pressure because the pipe string has been open at each of its ends, which affords little chance of any significant differential pressure developing between the outside and inside of the pipe. For this reason, conductor pipes are not typically designed to withstand pressure differentials that may exist across the conductor wall.
Trapped lubricant in the described prior art conductor pipe thread connectors has not been a limiting consideration. However, as wells have become more challenging, the probability of a “kick” occurring in the conductor pipe has increased. Such a kick may occur, for example, when the conductor pipe is driven into an abnormally pressured zone. This circumstance, as well as others, may require that the conductor pipe be designed to withstand and contain a high internal gas charge. Accordingly, conductor pipe used in offshore wells currently being drilled must be able to endure high, cyclic impact loads while maintaining a seal that can contain high pressures. This combination of problems is of recent origin, having a genesis coincident with the advent of deeper, more challenging offshore drilling.
Two major types of pressure seals are incorporated into the connections used to join large diameter conductor pipe: metal-to-metal radial seals and polymer O-rings. Some threaded large diameter connectors used in conductor pipe are equipped with an external polymer O-ring seal to restrict migration of seawater into the threaded area of the connection where it can cause corrosion or corrosion cracks to develop. Such corrosion in the connectors of conductor pipes is to be carefully avoided as it can behave as a stress riser that in turn may cause the fatigue life of the connection to be reduced.
In conductor seals intended to contain pressure, the metal-to-metal radial seal design is the preferred method of sealingly a conductor pipe. For this reason, metal-to-metal radial seals are commonly specified by the oil company customer for use in the construction of the conductor pipe.
While a metal-to-metal seal is a desirable feature for producing a pressure seal in conductor pipe, it is not universally found in such pipe. Moreover, while there are some conductor pipes that use an external elastomeric seal to restrict migration of seawater into the connection threads where it could cause corrosion or allow corrosion cracks to develop, the use of metal-to-metal seals for both an internal and external seal is not a common feature of conductor pipe design. Stress induced by the hydraulic loading of lubricant trapped in the connections of conductor pipe is a relatively new phenomenon resulting from the use of metal-to-metal internal and external seals. Accordingly, there has been little industry attention directed to the problem of pressurized thread lubricant trapped between internal and external seals on impact driven conductor pipe.
Metal-to-metal radial seal designs are well known in the broad field of oilfield tubulars. The metal-to-metal radial seal may be employed at one or both axially spaced ends of the engaged threaded area of the connection. The seal used to seal against the high-pressure fluids inside the tubular is generally referred to as an “internal” seal. A seal used at the opposite end of the engaged threaded connection used to seal fluids external to the threaded connection from entry into the threaded area is generally referred to as an “external” seal. Conductor pipe connections designed to provide pressure tight seals using both internal and external metal-to-metal seals have an increased probability of encountering damaging stresses produced as a result of having high-pressure trapped lubricant in the connections.
When the threaded connection has both an internal and an external seal, thread lubricant becomes trapped between the overlapping threaded areas of the connection as the connection is initially screwed together. This trapping action occurs as a result of the internal and external seals engaging before the threaded connection has been completely closed together.
The thread lubricant used for the threaded portions of the conductor connections is typically an oil-based grease compound containing solid particles. The thread lubricant compound is virtually non-compressible. Trapping an excessive amount of the thread compound between the internal and external seals can generate an enormous amount of hydraulic pressure between the pin and box as the volume containing the trapped fluid is decreased during the thread engagement process.
High impact loads applied during hammering of a conductor pipe are transferred across the engaged threads and can also produce high hydraulic pressures between the components of the connection. The impact induced hydraulic pressure in the thread lubricant is produced as a result of the required tight mechanical engagement between the mating threads of a conductor pipe connection. The tight thread engagement is necessary to resist relative movement between connected pipe sections during the repeated impacts driving the pipe into the earth. The tighter the connection, the more closely the connection resembles a solid tubular body which can best withstand the effects of the cyclic loading.
The stresses from impact loading as well as the stresses induced by high hydraulic pressures between the connection components can cause a failure of the threaded connection in a conductor pipe. Accordingly, stress control is extremely important in the design and construction of threads that must maintain a high degree of resistance to fatigue failure or other damaging effects of cyclic loading.
As contrasted with conductor pipe, a different set of performance and application requirements exists for the threaded connections of casing pipe that is run downhole into a well. Casing, unlike large OD conductor and conductor casing, is required to carry high loads but is not typically subjected to cyclic loading. Casing must also contain much higher pressures than are seen in conductor pipes. Pressure integrity must be maintained between the inside and outside of the casing. Typically, this pressure sealing is accomplished in the connections by incorporating metal-to-metal sealing elements. Simple tight fitting thread forms do not offer adequate sealing when the pressures are high, especially when high temperatures are present.
In many applications, “wedge” threads provide a superior connection for conductor pipe, casing and other tubulars. Casing connections with thread forms that wedge together have been available to the industry for over 20 years. The thread tooth on a wedge thread increases in width along its helical length. The gap between adjacent turns of a wedge thread tooth decreases in a direction opposite to the thread tooth increase. During assembly of wedge threads, the narrow end of the thread tooth of one component advances into an increasingly narrowing gap between thread teeth in the mating component. At full make-up, the thread tooth of one pipe component is wedged into the gap between thread tooth turns of a second pipe component to form a metal-to-metal engagement.
Wedge-type thread designs have not generally replaced traditional premium or high performance thread designs even though the wedging type thread forms offer superior torsional and compression strength. One reason for this lack of universal acceptance by the end users of the pipe is that the wedging thread forms have uncontrollable makeup characteristics. That is, the pin and the box of the connections rarely make up to the same final position relative to each other. This is a very detrimental characteristic when trying to engage metal seal elements on the pin and the box because these elements are typically designed on a slight taper. Therefore, if they are not accurately positioned relative to each other, they will not perform adequately when exposed to very high pressures.
The problem of unpredictable pin and box positioning when using wedging thread forms is due to several factors. One factor is the naturally occurring variations in the thread form shape along the entire helical length of the thread tooth body. This can be controlled to a certain extent by using high quality threading machines and special care in programming the machines. Another factor relates to the tight fit between the pin and box thread forms typical in connection designs that use wedging threads. This tight fit can entrap lubricant to the extent that the lubricant can't escape during the makeup procedure, which can cause a hydraulic layer of lubricant to be trapped between the pin and box threads. When this happens, the connection is further restricted from proper makeup position, which further hinders the proper engagement of the sealing elements.
A related problem resulting from trapping lubricant within a threaded connection is that the hydraulic forces in the lubricant resulting during makeup prevent the pipe assembly process from being accurately monitored. Most makeup monitoring systems evaluate the amount of torque being applied to a connection as a function of the relative rotation between the components being screwed together. The presence of trapped thread lubricant in the connection can produce an abnormal torque reading relative to the made up position of the connection.
The problem of trapped thread lubricant is dealt with, in part, in the connection of U.S. Pat. No. 4,830,411 (the '411 patent) by shaving the stab flank of the thread form to provide a helical flow passage that allows the escape of the trapped lubricant. The '411 patent also suggests providing a lubricant escape passage by removing a portion of the corner where the stabbing flank face and thread crest meet. Other thread features are also disclosed for preventing undesired makeup effects caused by trapping thread lubricant in the threaded area of the connection. The connection design of the '411 patent is intended for use in connections having an internal metal-to-metal seal and no external seal whereby the trapped lubricant may escape through the external, unsealed end of the connection. The thread modifications taught by the '411 patent can compromise the efficiency of a threaded connection, particularly those connections in which engagement of the thread flanks is an essential part of the thread design.
Another design directed toward stress reduction resulting from trapped lubricant is described in U.S. Pat. No. 6,050,610 (the '610 patent). The '610 patent suggests forming a groove in a thread root or at the intersection of a thread root with a thread flank to provide an escape passage for trapped lubricant. Formation of a recess in the root area of a thread can, however, concentrate stresses acting on the threaded connection, adversely affecting its strength.
Another prior art patent suggesting the use of grooves formed in a thread tooth or a thread root is U.S. Pat. No. 2,177,100 (the '100 patent). The ' 100 patent suggests the use of helical grooves in thread flanks or thread roots to provide line contact with the mating threads to provide a seal against root-to-crest-to-root leakage. Neither an internal nor an external metal-to-metal seal is proposed and lubricant contained in the grooves is said to provide a seal of the helically developed flow path formed by the grooves.